Depending on where you are from you can check Amazon and the likes. If you are in India like me, the options arr very limited. There is a case with the cut-outs but it is for the Tencent version. These also interfere with the glass protectors if you have one installed so that is not acceptable. So i have settled for the clear/black case with the corner bumps. Provides decent protection and takes care of the camera bump. They come with or without the cut out for the side port, though i prefer the one without it as you shouldn't have this case on if you are gaming/charging as it won't dissipate heat. Also since the phone is pretty big the cutouts on the side tend to make the case a bit 'flimsier'.
Amazon.com: Fanbiya Armor Case for ASUS ROG Phone 3 - TPU Frame Clear A-Grade Acrylic Back Compatibl...This is the case that I currently use. The padding on the sides seem to be decent, and it extends up a bit so you don't have to worry about the phone falling on the screen if you drop it. Back is clear so Aura RGB shows. There's space cut out for the Air Triggers, side port, air vent, and mics. Bottom USB-C port has a flap to cover it. Pretty nice, overall. It can be used with the AeroActive cooler, though it cannot close completely due to the padding. My only complaint is that it cannot be used with the TwinView Dock.